The committee’s 46-page report says the MOD is “gambling” on hitting efficiency targets, when the department has repeatedly failed to reach its own savings goals in the past.

Julian Lewis MP, chair of the committee, said: “It is extremely doubtful that the MOD can generate even more efficiencies from within its already stretched budget on the scale required to deliver its equipment plan.”

He said: “This will inevitably lead either to a reduction in the numbers of ships, aircraft and vehicles or to even greater delays in their acquisition.”

The report comes amid a row between the new Defence Secretary, Gavin Williamson, and the Chancellor over military funding and as a Cabinet Office review looks at redrawing defence and security priorities after a year of terror attacks and cyber hacks.

In the past, the MOD has proven incapable of making savings on the scale it is predictingCommons Defence Committee

But the MOD’s ability to afford the equipment plan rests on finding billions in efficiency savings that have yet to be found.

There is also growing doubt about the true scale of the savings needed, the committee found.

Figures set out in the SDSR suggested around £10bn, but Stephen Lovegrove, Permanent Secretary, has since put the figure at up to £30bn over the next decade.

The report found “uncertainty within the department over the true size and scale of the ‘efficiency savings’ required to keep the [equipment] plan and SDSR 2015 afloat”.

The report also highlights the department’s poor record in meeting efficiencies.

It said: “In the past, the MOD has proven incapable of making savings on the scale it is predicting and, even if all the ‘efficiencies’ are realised, there will be little room for manoeuvre in the absence of sufficient financial ‘headroom’ and contingency funding.

“This is not an adequate basis for delivering major projects at the heart of the UK's defence capability.”

An MOD spokesman said that the £178bn equipment plan continues to "deliver the cutting-edge kit to keep the UK safe".

He said: "As we told the Defence Committee, we are making good progress towards our efficiency target. We always look to provide the best value for money for the taxpayer, with all savings reinvested in defence.”